JavaAlgorithms/Others/QueueUsingTwoStacks.java
yanglbme 29948363da docs: update the whole repository
* fix some bugs
* delete duplicate files
* format code
2019-05-09 19:32:54 +08:00

161 lines
4.2 KiB
Java

package Others;
import java.util.Stack;
/**
* This implements Queue using two Stacks.
*
* Big O Runtime:
* insert(): O(1)
* remove(): O(1) amortized
* isEmpty(): O(1)
*
* A queue data structure functions the same as a real world queue.
* The elements that are added first are the first to be removed.
* New elements are added to the back/rear of the queue.
*
* @author sahilb2 (https://www.github.com/sahilb2)
*
*/
class QueueWithStack {
// Stack to keep track of elements inserted into the queue
private Stack inStack;
// Stack to keep track of elements to be removed next in queue
private Stack outStack;
/**
* Constructor
*/
public QueueWithStack() {
this.inStack = new Stack();
this.outStack = new Stack();
}
/**
* Inserts an element at the rear of the queue
*
* @param x element to be added
*/
public void insert(Object x) {
// Insert element into inStack
this.inStack.push(x);
}
/**
* Remove an element from the front of the queue
*
* @return the new front of the queue
*/
public Object remove() {
if(this.outStack.isEmpty()) {
// Move all elements from inStack to outStack (preserving the order)
while(!this.inStack.isEmpty()) {
this.outStack.push( this.inStack.pop() );
}
}
return this.outStack.pop();
}
/**
* Peek at the element from the front of the queue
*
* @return the front element of the queue
*/
public Object peekFront() {
if(this.outStack.isEmpty()) {
// Move all elements from inStack to outStack (preserving the order)
while(!this.inStack.isEmpty()) {
this.outStack.push( this.inStack.pop() );
}
}
return this.outStack.peek();
}
/**
* Peek at the element from the back of the queue
*
* @return the back element of the queue
*/
public Object peekBack() {
return this.inStack.peek();
}
/**
* Returns true if the queue is empty
*
* @return true if the queue is empty
*/
public boolean isEmpty() {
return (this.inStack.isEmpty() && this.outStack.isEmpty());
}
}
/**
* This class is the example for the Queue class
*
* @author sahilb2 (https://www.github.com/sahilb2)
*
*/
public class QueueUsingTwoStacks {
/**
* Main method
*
* @param args Command line arguments
*/
public static void main(String args[]){
QueueWithStack myQueue = new QueueWithStack();
myQueue.insert(1);
System.out.println(myQueue.peekBack()); //Will print 1
// instack: [(top) 1]
// outStack: []
myQueue.insert(2);
System.out.println(myQueue.peekBack()); //Will print 2
// instack: [(top) 2, 1]
// outStack: []
myQueue.insert(3);
System.out.println(myQueue.peekBack()); //Will print 3
// instack: [(top) 3, 2, 1]
// outStack: []
myQueue.insert(4);
System.out.println(myQueue.peekBack()); //Will print 4
// instack: [(top) 4, 3, 2, 1]
// outStack: []
System.out.println(myQueue.isEmpty()); //Will print false
System.out.println(myQueue.remove()); //Will print 1
System.out.println(myQueue.peekBack()); //Will print NULL
// instack: []
// outStack: [(top) 2, 3, 4]
myQueue.insert(5);
System.out.println(myQueue.peekFront()); //Will print 2
// instack: [(top) 5]
// outStack: [(top) 2, 3, 4]
myQueue.remove();
System.out.println(myQueue.peekFront()); //Will print 3
// instack: [(top) 5]
// outStack: [(top) 3, 4]
myQueue.remove();
System.out.println(myQueue.peekFront()); //Will print 4
// instack: [(top) 5]
// outStack: [(top) 4]
myQueue.remove();
// instack: [(top) 5]
// outStack: []
System.out.println(myQueue.peekFront()); //Will print 5
// instack: []
// outStack: [(top) 5]
myQueue.remove();
// instack: []
// outStack: []
System.out.println(myQueue.isEmpty()); //Will print true
}
}